The long-term research objective is to design, synthesize and investigate model compound systems which can help elucidate fundamental aspects of structure, metal-ligation, spectroscopy and reactivity relevant to the dioxygen (O2) and nitrogen oxide (NOx) chemistry utilized by heme-copper oxidases (HCOs), nitric oxide reductases (NORs) and related proteins. HCOs and NOR's are evolutionarily related enzymes which play critical roles in cellular processes within aerobic and anaerobic organisms. They have in common a heme/M (M = Cu or non-heme Fe) active site that reductively cleaves O2 or NO, respectively. The research proposed will contribute to a better understanding of enzyme structure and mechanism by providing a comprehensive and fundamental basis relevant to biological processing of O2, NO and nitrite (NO2-), that extends even beyond the heme and Cu metalloprotein sphere. Specific aims include (1) the characterization of heme/Cu- O2 adducts, new protonated and low-spin derivatives and elucidation of their (structures) and electronic/bonding properties, (2) the study of reductive O-O cleavage promoted by heme-peroxo-Cu complex systems, especially new low-spin compounds where altering the copper-ligand environment and heme axial 'base' ligands is a part of the approach. Systematically varied reducing agents and acids will be employed in the investigations in order to elucidate those factors crucial for this process, comprising a critical aspect of 'oxygen activation' in chemical or biochemical systems, (3) the employment and study of ligand systems which possess a copper-ligand imidazole-phenol moiety which induces O-O reductive cleavage chemistry for a corresponding heme/copper-O2 assembly. Also, two chemical systems designed to test how CcO (bio)chemistry leads to the actual formation of the copper-ligand His-Tyr crosslink, will be studied in detail. (4) investigation of a chemical system with input variations f the heme axial 'base' ligand, where heme/NO/O2 coordination chemistry will be studied mechanistically with regard to peroxynitrite formation and its subsequent reactivity. This chemistry occurs in NO dioxygenases (NODs), enzymes critically involved in cellular NO homeostasis and possibly in cellular signaling via amino-acid nitration chemistry. (5) the study of chemistry relevant to NORs, heme/Cu assemblies that enable NO reductive coupling. A clear focus will be on the mechanism of formation of putative hypontrite intermediates, their structures and their reactivity leading to N2O and H2O products. Such information is key to the understanding of N-N coupling and N-O cleavage chemistries which are also coupled to protonation. These processes are also of broad interest with respect to other biological metalloenzyme nitrogen oxide processing. This aim also includes efforts to elucidate the chemistry of heme/Cu mediated nitrite reduction to NO and complementary NO oxidation to nitrite. In-hand heme/Cu assemblies enable this chemistry and further mechanistic probing is necessary; these processes are critical to NO signaling and linked to cellular responses to changes in [O2] concentrations.